insulin resistance and metabolic syndrome

8/11/2019 Insulin Resistance and Metabolic Syndrome

1/8

This document has been created with TX Text Control Trial Version 13.0 - You can use this trial v ersion for further 57 days.

5. INSULIN RESISTANCE AND METABOLIC SYNDROME

Istvn Wittmann

Second Department of Medicine, University of Pcs, Medical Faculty, Pcs, Hungary

1. Introduction

Combination of metabolic abnormalities in patients with high vascular risk is well known since

long time. In his Banting lecture Reaven used the term Syndrome X for the description of

the coexistence of these metabolic diseases. Other names as e.g. Deadly Quartet and insulin

resistance syndrome emphasised the consequences and the possible cause of this syndrome.

Most of the authors accept that the syndrome in the patients induces serious complications and

that in its background insulin resistance could be the major determinant.

Nevertheless, recently the existence of metabolic syndrome was questioned and an independenttreatment of the components of the metabolic syndrome was suggested. Nowadays, in this

post metabolic syndrome period newest papers are examining new and old questions,

whether metabolic syndrome exists, what are the important components, is diabetes mellitus a

component or a final result of the syndrome, which definition describes better the essence of

the syndrome, which definition predicts the mortality, which components are better predictors

for cardiovascular morbidity and mortality, etc. However, a relatively new estimation formula

for insulin resistance using homeostasis model assessment (HOME) made possible to study the

role of insulin resistance in big cohorts.

2. Insulin resistance and definitions of metabolic syndrome

The first international definition was given by the World Health Organization (WHO) in 1999.

This was focusing on the glucose intolerance and insulin resistance. Other components as

elevated blood pressure (> 140/90 mmHg), raised plasma triglycerides (> 1.7 mmol/l),

decreased HDL cholesterol (for men < 0.9 mmol/l, for women < 1.0 mmol/l), obesity

(waist-hip ratio for men > 0.9, for women > 0.85; and/or BMI > 30 kg/m2), and

microalbuminuria (urinary albumin excretion rate > 20 g/min or albumin:creatinine ratio > 30

mg/g) were also involved.

The European Group for the Study of Insulin Resistance (EGIR) in 1999 proposed a definitionto be used in non-diabetic patients only and put the insulin resistance in the centre, as well.

In 2001 the National Cholesterol Education Program Third Adult Treatment Panel (ATP III)

set a new definition concentrating rather on the obesity and dyslipidaemia. Three or more of

the following risk factors are needed for the diagnosis of metabolic syndrome: fasting plasma

glucose > 6.1 mmol/l, blood pressure > 130/85 mmHg, triglycerides > 1.7 mmol/l, HDL

cholesterol for men < 1.03 mmol/l, for women < 1.29 mmol/l, waist circumference for men >

102 cm, for women > 88 cm.

In 2002 the American Association of Clinical Endocrinology (AACE) presented a position

statement, again stressing the role of insulin resistance in the syndrome.


2/8


In 2005 the International Diabetes Federation (IDF) offered a consensus worldwide definition

of the metabolic syndrome. This document shows the complexity of the problem of metabolic

syndrome and divides the definition in four parts: part 1 contains definition for use of clinical

practice, part 2 gives the additional metabolic criteria for research, part 3 details the

recommendations for treatment, and part 4 suggests some future works. The definition for use

in clinical practice is similar to the ATP III criteria focusing on the central obesity, but stillkeeping type 2 diabetes in the definition of the syndrome and strongly recommends the oral

glucose tolerance test (OGTT). On the other hand, central obesity and insulin resistance were

acknowledged equally important causative factors. Thus, IDF definition could be characterized

as a mixture of WHO and ATP III criteria. According to this definition patients with

metabolic syndrome must have central obesity defined as waist circumference > 94 cm for

Europid men, and > 80 cm for Europid women plus any two of the following factors: raised

triglycerides > 1.7 mmol/l or specific treatment for this lipid abnormality, reduced HDL

cholesterol < 1.03 mmol/l in males and < 1.29 mmol/l in females or specific treatment for this

abnormality, raised blood pressure > 130/85 mmHg or specific treatment for hypertension,

raised fasting plasma glucose > 5.6 mmol/l or previously diagnosed type 2 diabetes mellitus. Inthe part of additional metabolic criteria for research are as follows: abnormal body fat

distribution, atherogenic dyslipidemia (beyond elevated triglyceride and low HDL cholesterol),

dysglycemia determined by OGTT, insulin resistance measured by fasting insulin/proinsulin

ratio, HOMA, minimal model, elevated free fatty acids or clamp, vascular dysregulation

(endothelial dysfunction and microalbuminuria), prothrombotic state, and hormonal factors. In

the part of recommendation for treatment, management of insulin resistance and hyperglycemia

was declared. Metformin, thiazolidindions, acarbose, orlistat, incretin mimetics, dipeptidyl

peptidase IV inhibitors, protein tyrosine phosphates 1B inhibitors, and the endocannabinoid

receptor blocking agents are listed as verified or future therapeutic possibilities.

In 2005, after the IDF definition, the American Heart Association and the National Heart,Lung, and Blood Institute (AHA/NHLBI) gave an up-to-date guidance for the diagnosis and

treatment of metabolic syndrome in adults. It accepted the existence of the metabolic

syndrome, kept all of the ATP III criteria except of the fasting plasma glucose, which was

decreased to 5.6 mmol/l.

3. Pathogenesis of insulin resistance in metabolic syndrome

Confirmatory factor analysis of Spanish, Mauritian, and U.S. populations was carried out to

search for a single common factor of metabolic syndrome. This study confirmed the current

clinical components of the metabolic syndrome (insulin resistance, waist circumference,triglyceride, HDL cholesterol, blood pressure). Leptin and uric acid were suggested to be

possible contributors, and a single latent, common factor underlying in the background was

supposed.

Candidate causative factors are visceral obesity, elevation of circulating free fatty acids (FFA),

subclinical inflammation, production of reactive oxygen species, endothelial dysfunction,

decrease of the first phase insulin secretion, smoking, etc.

Role of visceral obesity and/or FFA was recently discussed. Computer tomography (CT) scans

and MRI examinations verified intra-abdominal, predominantly mesenteric and omental,adiposity as independent predictors of insulin resistance. On the other hand, contradictory data


3/8


are available for the role of subcutaneous adiposity in insulin resistance. Removal of abdominal

subcutaneous adipose tissue by liposuction, in turn, does not improve insulin resistance, and

treatment using thiazolidindions (so called insulin sensitizers) increases subcutaneous fat mass

and insulin sensitivity. Lower body obesity and Prader-Willy syndrome with normal visceral

adipose tissue mass seems not to induce insulin resistance. Others rather stress the role of FFA

in the development of insulin resistance suggesting that this could be the most importantcontributor, and visceral obesity might not be the major factor because it is responsible for only

20-25% of total FFA delivery to the liver and less than 5% for the systemic (extrahepatic) FFA

availability.

Several lines of evidence support the pivotal role of subclinical inflammation and production of

cytokines in the development of insulin resistance. One of the key features is the induction of

intracellular formation of reactive oxygen species (ROS) by cytokines. Cytokines-induced ROS

production decreases insulin-mediated glucose uptake of the fat cells. This could be prevented

by antioxidants, e.g. N-acetylcystein, superoxide dismutase (SOD), and catalase. This effect

could be mimicked by the overexpression of catalase in the cytosol and in the mitochondria,and overexpression of CuZnSOD and MnSOD in these fat cells. TNF-decreases intracellular

insulin signal transduction, using a ROS dependent pathway, at least partially by the inhibition

of serin phosphorylation of Akt (protein kinase B, PKB), which is an intracellular mediator of

the metabolic effect of insulin.

Akt is not only a transducer of the insulin effect in the fat cells, but it is a very important

regulator of the vascular tone, as well. Insulin induces activating phosphorylation of Akt, and

this phospho-Akt (P-Akt) evokes activating phosphorylation of endothelial nitric oxide

synthase enzyme (eNOS), which produces nitric oxide and results in vasodilation. This way

insulin dilates nutritive precapillary arterioles increasing circulation of parenchyma, and leading

to the metabolic effect of insulin, and decreases blood pressure preventing target organdamage. In insulin resistant state due to the overproduction of cytokines and ROS, insulin is

not able to induce full vasodilation anymore (endothelial dysfunction) because of the decrease

of P-Akt. Diminished circulation in the nutritive capillaries and inhibited intracellular signalling

in the parenchymal cells lead to insulin resistance and hypertension. In this stage secondary

hyperinsulinemia due to the second phase insulin hypersecretion could augment sympatethic

nervous system and renin-angiotensin system activity causing further elevation of blood

pressure.

Some authors suggest the primary role of the decrease of the first (acute) phase of the insulin

secretion in the insulin resistance, since this develops as the impaired glucose tolerance (IGT)or obesity are present, and decrease of first phase insulin secretion causes early postprandial

hyperglycemia. This, in turn, results in the elevation of the second phase insulin secretion,

mentioned before.

There are papers suggesting endothelial dysfunction as a primary abnormality in the

development of insulin resistance. This is based on the processes detailed above, and on the

observations proving that in some cases endothelial abnormalities precede insulin resistance.

The role of decrease of first-phase insulin secretion in the insulin secretion in the insulin

resistance, blood pressure and target organ damage ispresented in Figure 5.1.).


4/8


Figure 5.1.The role of decrease of first-phase insulin secretion in the insulin resistance, blood pressure and

target organ damage.

Current smoking is an independent predictor for metabolic syndrome. Even passive smokers

have an elevated prevalence of metabolic syndrome compared to the never smoker population.

Furthermore, adult offspring of smoker parents, exposed to their parents smoking during their

childhood have a significantly elevated risk for the development of metabolic syndrome. Insmokers the value of HOMA is higher than in matched controls suggesting insulin resistance.


5/8


In lean current smokers an increase of the visceral adiposity could be detected. According to

our, not published, results smoke induces a concentration and a time dependent decrease of the

phosphorylation of the Akt.

4. Cardiovascular risk and insulin resistance

In the last two years more publications compared the predictive value of the different

definitions of metabolic syndrome or carried out a direct comparison of insulin resistance with

metabolic syndrome. WHO-definition was supposed to be more representative for the insulin

resistance, than ATP III or IDF.

In a prospective cohort study authors investigated the cardiovascular risk of metabolic

syndrome and insulin resistance. Patients were examined by coronary angiography. The main

outcome was the incidence of vascular events during 2.3 year follow up. Hazard ratio (HR) for

vascular events was 2.74 in case of metabolic syndrome (ATP III) and 1.51 in case of insulin

resistance (both were significant). They concluded that insulin resistance is a strong and

independent predictor of vascular risk.

In the Uppsala Longitudinal Study of Adult Men the role of insulin sensitivity in the

development of congestive heart failure (CHF) was examined. In a population of elderly men

(> 70 years) the first hospitalization for CHF was detected during a follow-up of 8.9 years.

Risk of CHF was associated in a multivariate analysis with 2-hour glucose, fasting serum

proinsulin level, body mass index, and waist circumference. If clamp glucose disposal rate was

added to the model, obesity variables were no longer predictors for CHF. According to these

results authors supposed, that the earlier described association between obesity and CHF may

be mediated by insulin resistance.

The DECODE Study Group compared WHO, ATP III, modified ATP III (fasting plasma

glucose was lowered from 6.1 mmol/l to 5.6 mmol/l), and IDF definitions of metabolic

syndrome in predicting cardiovascular disease (CVD). HR was higher in case of WHO

compared to the others in the male patients, but all definitions had a low predictive value in the

female population.

In the British Womens Heart and Health Study the general finding was the same as

in the DECODE Study.

In a Canadian long-term (12.6 years) study in a mainly male (75.6%) population HR

for all-cause mortality was 1.2 using ATP III and 1.56 using WHO definitions.

In another prospective, population based cohort study ATP III gave a long termprognostic information about total and cardiovascular mortality.

Taken together, these studies suggest that the WHO definition may be better in predicting the

cardiovascular diseases. This definition system is based mainly on the insulin resistance

concept. On the other hand insulin resistance seems to be an independent predictor of the CVD

morbidity and mortality.

5. Recent studies using insulin sensitizers

Thiazolidindions are agonists of peroxisome proliferators-activated receptor(PPAR). Thesedrugs increase hepatic and peripheral insulin sensitivity, preserve insulin secretion and


6/8


pancreatic-cell function. There are some data about their beneficial effect on the subclinical

inflammation, as well. Through all these effects these drugs could decrease the risk of

development of diabetes mellitus and macrovascular complications. Troglitazone, the first

member of this group of drugs was withdrawn because of hepatotoxicity. Pioglitazone and

rosiglitazone are now on the market.

In the Diabetes Reduction Assessment with ramipril and rosiglitazone Medication (DREAM)

Trial effect of rosiglitazone on the frequency of diabetes type 2 was examined. In this study

patients with impaired glucose tolerance (IGT), with impaired fasting glucose (IFG) or both

were involved. They were obese, hypertensive. These three parameters (carbohydrate

abnormality, obesity, and elevated blood pressure) indicate that these patients could be mainly

metabolic syndrome patients. They were followed for 3.0 years. The rosiglitazone group was

compared to a placebo group. The primary outcome was a composite of incident diabetes

mellitus and death. Rosiglitazone decreased the risk of composite primary outcome by about

60%, the risk for incident diabetes mellitus by 62%, and increased the regression from IGT or

IFG or both to the normal glucose tolerance by 71% (fasting plasma glucose < 6.1 mmol/l) andby 83% (fasting plasma glucose < 5.6 mmol/l). The drug was more effective in the Indian and

in the more obese population. Rosiglitazone significantly decreased the blood pressure, by 1.7

mmHg of the systolic and by 1.4 mmHg of the diastolic blood pressure. These results support

the general approach of the insulin resistance detailed above emphasizing the role of Akt,

eNOS and the endothelium in the development of metabolic syndrome. There is another

interesting conclusion of this trial, namely that the treated group had an increase of body

weight, body mass index, waist-to-hip ratio and waist circumference. It could mean, that

rosiglitazone decreases insulin resistance in spite of the increase of body weight, i.e. eliminates

the relation between obesity and insulin resistance. Rosiglitazone did not influence the

cardiovascular endpoints but the incidence of non-fatal CHF was increased.

In a secondary prevention study in the PROspective pioglitAzone Clinical Trial In

macroVascular Events (PROactive) effect of pioglitazone on the macrovascular morbidity and

mortality was investigated. In type 2 diabetic, high risk patients the primary endpoint was the

composite of all-cause mortality, non-fatal myocardial infarction, stroke, acute coronary

syndrome, endovascular or surgical intervention in the coronary or leg arteries, and amputation

above the ankle. The secondary endpoint was the composite of all-cause mortality, non-fatal

myocardial infarction, and stroke. The time of observation was 34.5 months. Pioglitazone

treatment did not influence the primary endpoint, but significantly decreased the secondary

endpoint by 16%. Systolic blood pressure was significantly decreased by 3 mmHg. On the

other hand, in the treatment arm the incidence of non-fatal CHF significantly increased.

These two big trials with insulin sensitizers suggest that thiazolidindions could have a

preventive effect in the progression of insulin resistance, but their cardiovascular effectiveness

should be further investigated.

6. In conclusion

Metabolic syndrome definitions try to identify patients at high risk for cardiovascular diseases.

WHO definition relies on insulin resistance, ATP III is based rather on the obesity. IDF

definition is a mixture of WHO and ATP III. Insulin resistance could be in the centre of thepathogenesis of metabolic syndrome. Overproduction of cytokines and reactive oxygen species


7/8


could play a role in the insulin resistance. Importance of decrease of first phase insulin

secretion and endothelial dysfunction in the development of insulin resistance could be also not

excluded. In the majority of cases cardiovascular endpoints are better predicted by the insulin

resistance-based WHO definition, than by the others. Insulin resistance itself is a predictor of

cardiovascular morbidity and mortality. Drugs affecting directly the insulin resistance (the so

called insulin sensitizers) prevent the progression of insulin resistance and this way decreasethe rate of development of type 2 diabetes in IGT and IFG patients. This effect of insulin

sensitizers evolves in spite of their increasing effect on the body weight. Effects of insulin

sensitizers on the cardiovascular morbidity and mortality remain to be elucidated.

Recommended literature:

1. Dormandy JA, Charbonnel B, Eckland DJ et al. Secondary prevention of macrovascular events in

patients with type 2 diabetes in the PROactive Study (PROspective pioglitAzone Clinical Trial In

macroVascular Events): a randomised controlled trial. Lancet 2005; 366:1279-89.

2. Saely CH, Aczel S, Marte T et al. The metabolic syndrome, insulin resistance, andcardiovascular risk in diabetic and nondiabetic patients. J Clin Endocrinol Metab 2005;

90:5698-703.

3. Weitzman M, Cook S, Auinger P et al. Tobacco smoke exposure is associated with the metabolic

syndrome in adolescents. Circulation 2005; 112:862-9.

4. Lebovitz HE, Banerji MA. Point: Visceral adiposity is causally related to insulin resistance.

Diabetes Care 2005; 28:2322-4.

5. Miles JM, Jensen MD. Counterpoint: visceral adiposity is not causally related to insulin

resistance. Diabetes Care 2005; 28:2326-8.

6. Alberti KGMM, Zimmet P, Shaw J. Metabolic syndrome a new world-wide definition. A

consensus statement from the International Diabetes Federation. Diabet Med 2006; 23:469-80.7. The DREAM (Diabetes Reduction Assessment with ramipril and rosiglitazone Medication) Trial

Investigators. Effect of rosiglitazone on the frequency of diabetes in patients with impaired

glucose tolerance or impaired fasting glucose: a randomised controlled trial. Lancet 2006;

368:1096-105.

8. Lawlor DA, Smith GD, Ebrahim S. Does the new International Diabetes Federation definition of

the metabolic syndrome predict CHD any more strongly than older definitions? Findings from the

British Womens Heart and Health Study. Diabetologia 2006; 49:41-8.

9. Nigam A, Bourassa MG, Fortier A et al. The metabolic syndrome and its components and the

long-term risk of death in patients with coronary heart disease. Am Heart J 2006; 151:514-21.

10. Sundstrm J, Riserus U, Byberg L et al. Clinical value of the metabolic syndrome for long term

prediction of total and cardiovascular mortality: prospective, population based cohort study. BMJ

2006; 332:878-82.

11. The DECODE Study Group, Qiao, Q. Comparison of different definitions of the metabolic

syndrome in relation to cardiovascular mortality in European men and women. Diabetologia

(DOI 10.1007/s00125-006-0438-6)

12. Hunt KJ, Hansis-Diarte A, Shipman K et al. Impact of parental smoking on diabetes,

hypertension and the metabolic syndrome in adult men and women in the San Antonio Heart

Study. Diabetologia 2006; 49:2291-8.

13. Ingelsson E, Sundstrm J, Arnlw J, et al. Insulin resistance and risk of congestive heart failure.

JAMA 2006; 294:334-41.

14. Houstis N, Rosen ED, Lander ES. Reactive oxygen species have a casual role in multiple forms

of insulin resistance. Nature 2006; 440:944-8.


8/8


15. Pladevall M, Singal B, Williams LK et al. A single factor underlies the metabolic syndrome.

Diabetes Care 2006; 29:113-22.

insulin resistance and metabolic syndrome

Documents